Object controlling servo system



P. HALPERT ETAL OBJECT coNTRoLLrNG sERvo SYSTEMv July 29, 195s P.HALPl-:R'r ET'AL 2,845,239

OBJECT coNTRoLuNG sERvo SYSTEM v original Filed May e, 1949 2sheets-sheet 2 July 29, 195s United States Patent() OBJECT CONTROLLINGSERVO SYSTEM Percy Halpert, Hempstead, and Robert S. Curry, Jr.,Baldwin, N. Y., assignors to Sperry Rand Corporation, a corporation ofDelaware Original application May 6, 1949, Serial No. 91,844. Di-Xisddand this application February 8, 1955, Serial No.

9 Claims. (Cl. 244-77) This invention relates to object controllingservo systems of a type in which the object is movable about an axis dueto disturbing moments of transient and persistent characters and is adivision of our copending application Serial No. 91,844, iiled-May 6,1949, now abandoned, and assigned to the same assignee as the presentapplication. The invention is particularly adapted for use in automaticpilots for dirigible craft or aircraft where the craft is the object andthe crafts control surface is the controller which facilitatescorrective movement of the object about one of its axes.

The present invention is particularly concerned with the reduction inthe magnitude of the displacement error in a system -of this characterwhen a persistent disturbing moment about the axis of the movable objector craft is encountered. The term displacement error will be understoodtomean the departure in the aircraft attitude from a reference attitudeestablished prior to the sustained disturbance in aircraft trim. Errorsof this type are inherent in all systems where displacements from areference position are used to generate control signals, and thesesignals acting through a suitable control system produce restoringforces or moments on or yabout the object or craft in a sense to restorethe object or craft to its reference position. Persistent disturbingforces or moments acting to move the object or craft require a sustaineddisplacement of the object or craft from its reference position togenerate an error signal which acting through the control system willproduce an equal but opposing force or moment. These persistentdisturbances can be either external or internal to the control system.The object of the present invention is to eliminate this characteristicdisplacement error so that the object or craft can be restored to itsreference position.

A further object -of the invention is to reduce the displacement errorcreated in automatic pilots employing position feedback in the controlsurface servo system. In this case, a displacement of the object orcraft from its reference position is required to compensate for.position feedback signal arising from a new control surface position. i

A still further object of the invention is to obtain the same result inautomatic pilots for aircraft employing both a main control surface anda trim tab for moving the same about an axis. In this form of theinvention, the improved automatic pilot functions to relieve sustainedcable eifort on the main control surface of the craft so that theautomatic pilot may be disengaged with safety at any time.

Other objects, featuresand structural details of the invention will beapparent from the following description when read in relation to theaccompanying drawing, wherein: l

Fig, l is a side elevation of an object in the form of an aircraftemploying a servo system or automatic pilot in which the presentinventive concepts are incorporated;

A 2,845,239 Patented July 29, 1.958

yffice Figs. 2 and 3 are curves usedin explaining the operation of thesystem shown in Figs. 4 yand 4A;

Fig. 4 is a circuit diagram and schematic View of a servo system orautomatic pilot embodying the present inventive concepts;

. Fig. 4A is a view similar to Fig. 4 in which the system isparticularly directed to automatic pilot for aircraft employingdisplacement repeatback means in the servo system;

Fig. 5 is a view similar to Fig. 4 showing a modified form of theinvention; and

Fig. 6 is a further view similar to Fig. 4 showing .a furthermodification of the invention as particularly` directed to automaticpilots for aircraft employing both a main control surface and a trimtabfor moving the same about an axis. p

With particular reference to the form of the invention illustrated inFig. 4, the improved servo system or automatic pilot is shown to includemeans for moving a body in the form of an aircraft 11 about its pitch orlateral axis. As shown, the moving means may be the control surface orelevator 10 of the craft 11 together with va motor 12 `operativelyconnected to the control surface by way of shaft 13, suitable reductiongearing, drum 14 and cables 15. Primary signal means are also providedfor operating the moving means with displacement of the object or craft11 from a reference position about its lateral axis in this instance.The signal means illustratively shown herein is a selsyn transmitter orother pick-off 16 which may be located at' the pitch or inner gimbalaxis of a gyro Vertical 17. As shown, the single phase Wound rotor ofpick-off 16 may be stabilizedin pitch by mounting the same in fixedrelation to the gyro rotor case or frame 18 of the gyro vertical. Thethree phase Wound stator of the pick-off 16 may be xedly mounted on thering 20 ofthe gyro vertical in a wellknown manner. The gyro vertical ismounted in the craft so as to provide a reference for detecting changesin attitude of the craft about its pitch axis, Y`in this instance. Itwill be understood vthat other equivalent means may be used to positionone of the partsof a pick-off soy as to detect displacement of the craftor any object from a reference position about any desired axis thereof.The rotor of pick-off 16 is energizedfrorn a suitable source ofalternating current electrical energy.

When the craft or object is atlits reference position, the stator ofpick-off 16 in conjunction with stator of the monitoring transmitter orsynchro device 26 provides a null output from its rotor 27. With adeparture of the object or craft from its reference position, thepick-off 16 provides a control signal output from rotor27 for operatingmotor 12 whose direction of rotation depends `on the direction of suchdeparture and whose magnitude depends on the amplitude of thedisplacement. n

As herein shown, the signal of synchro device 26 is supplied to motor 12by way of an amplier 21 vofcouventional form which in the servo systemor automatic pilot illustrated is arranged to operate the elevator motorthrough the agency of a motor-generator set of the Ward- Leonard type.Thus, as shown, the output of amplifier 21 energizes the iield circuitsof a direct current generator 22, the armature circuit of whichenergizes the armature of motor 12 by way of leads 23 and Z4. Generator22 is driven by a motor 25 that is rotatingat a constant speed. The D.C. power supplied to motor 25 also energizes the iield windings of motor12, as shown. In the illustrated embodiment of the invention, it will beunderstood that the gyro vertical is mounted in craft 11 with its minoraxis parallel to or coincident with the pitch axis of the craft. v

In order to maintain the craft in a trim condition about its pitch axisfor a given loading, the effective output 3. of pick-off 16 is nullledby the synchro device 26 whose rotor 27 is in the circuit including theamplifier 21. This is to permit the craft to tiy at a slightly differentangle than that required by the reference instrument 17 and pick-off 16.As shown, the stator of synchro device 26 is fed from the output ofpick-off 16 by way of leads 28, 29 and 30. The rotor 27 of device 26 maybe positioned by means of a motor 31 that is energized by the output ofan amplifier 32 that is connected to the motor by way of a double pole,double throw switch 33. In this form of the invention, the input foramplifier 32 is obtained from a circuit including leads 34 and 35 thatare in parallel with the input circuit to amplifier 21.

In synchronizing the system prior to engaging the control for automaticflight, switch 33 is set in the dotted line position shown in Fig. 4 andthe craft, in this instance, is flown at the desired attitude. Anyoutput from pickoff 16 to amplifier 21 under such conditions reflects inan input to amplifier 32 whose output drives motor 31 so that the outputof the combined pick-off 16 and device 26 is zero. This defines areference position about the pitch axis of the craft and departure fromthis position is detected or measured by pick-off 16. When the servosystem or automatic pilot is engaged, switch 33 is simultaneously thrownto its full position in Fig. 4.

The conventional form of automatic pilot may further include means forchanging the reference attitude of the aircraft after the automaticpilot is engaged. Such means is shown in the drawing by a manuallysettable knob 36, movable arm 37, connected to the knob andpotentiometer 38 of which the movable arm is a part. The potentiometermay be energized from a suitable source of alternating currentelectrical energy by way of transformer 40. The potentiometer 38 isincluded in series with the rotor of device 26 in the input circuit toamplifier 21. In synchronizing the system or automatic pilot with switch33 in its dotted line position in Fig. 4, although contrary to goodpractice, it will be understood that knob 36 need not be located in itscentral position since rotor 27 will be positioned to produce an equalbut opposing signal to nullify the signal from potentiometer 38.

The elements of the servo system or automatic pilot described to thispoint are well known. In operation as a result of disturbing moments ofa transient character, the system functions by deriving a control signalfrom pick-off 16 as the craft or object departs from its referenceposition, creating a moment that is effective to restore the craft orobject to its reference position. The concept of frequency response isused in the subsequent discussions because this method of evaluating acontrol system is used most commonly. This concept has long been used inthe radio and telephone arts to express the fidelity of response ofequipment, and as a result a considerable background of establishedmathematics and experimental procedure has been made available toinstrument and automatic control engineers.

A transient analysis of the response of an automatic control to somestandard type of disturbance is an accepted procedure. However, with themore complex systems it becomes unwieldly, and useful design criteriabecome increasingly difficult to develop. It can be shown that afrequency analysis based on the mathematics of the Fourier series andthe Fourier integral provides information equivalent to the transientanalysis but with much less calculation and with more easilyinterpretable results.

With reference to Fig. 2, the abscissa scale represents the frequency ofthe disturbing moment measured in terms of cycles per second ofsinusoidal motion of the craft or object about some principal axis. Forpurposes of the present application, disturbing moments created by roughair action will be considered as one source of craft disturbance.Experience has shown that rough .4, air transients can be simulated bysinusoidal disturbing moments having frequencies within the band rangingfrom .1 cycle per second to 4.0 cycles per second. The system orautomatic pilot of the present invention functions in conventionalmanner when responding to disturbances that fall within this band. Wherethe disturbing moment has a frequency that is less than .1 cycle persecond, it is herein termed a persistent moment. The ordinate scale ofthe curves in Fig. 2 represents the output signal of the amplifier 21which is the controlling voltage of the system. Heretofore, the controlvoltage of the system varied with the frequency of the sinusoidaldisturbing moment as indicated by curve C of Fig. 2.

As exemplary of a moment whose frequency is less than .1 cycle persecond, it will be assumed that a craft flying in the directiondesignated by the horizontal arrow D in Fig. l, has a bomb load releasedfrom aft of the center of gravity of the craft. This change in loadingin general will cause the center of gravity to shift further forwardthan the shift in either the center of lift or center of pressure,thereby creating a nose down pitching moment W as indicated in Fig. l.As shown in Fig. l, this persistent moment is balanced by operation ofthe system or automatic pilot to maintain elevator 10 in an up positionso that a balancing moment is exerted about the center of gravity of thecraft whose force is indicated at E. This requires a continuous upelevator signal from the automatic pilot which is obtained from thepickoff 16 by reason of the fact that the nose of the craft is thenpointed downward as indicated by arrow F, and the craft continues to flyin the direction of arrow D in an out of trim condition, since asustained error is required to maintain a sustained control correction.With such a condition, the signal from pick-off 16 does not restore thecraft to its reference position about its pitch axis. In this instance,the pick-off 16 maintains an output from amplifier 21 to locate thecontrol surface or elevator 10 displaced from its normal or centralposition. This system thus operates under conditions of a persistentdisturbing moment balanced by a control moment about the axis of thecraft with a herein termed displacement error signal from pick-off 16.At such time, it will be understood that the attitude of the craft aboutits pitch axis is displaced from its reference position so that theoutput from pick-off 16 is obtained.

In accordance with the teaching of the present invention, means areprovided for producing an additional signal for operating motor 12 whichin this form of the invention effectively supplements the displacementerror signal of the primary pick-off 16. As shown in Fig. 4, such meansfor operating motor 12 of the system is in the form of a circuit, theoutput of which is monitored by the signal of pick-off 16. This circuitincludes amplitier 32 and an integrating network indicated at 41provided, in this instance, by resistors 42, 43 and condensers 44, 45.The direct current output of the network 4l, taken across capacitor 45and impressed across rcsistor 46 is in series in the input circuit toamplifier 21. The network 41 is included in the system when switch 33 isin its normal full line position as shown in Fig. 4.

The output of amplifier 21 due to this additional signal means in thesystem is indicated by curve G in Fig. 2. This control signal is onlyeffective with persistent disturbing moments about the axis of thecraft. With disturbances above .l cycle per second, the output of thisradditional signal means is relatively small and therefore does notinterfere with the noted functioning of the system at such time. Thedouble integrating network is provided in this instance to obtainsharper cut-off characteristics. Curve R in Fig. 2 shows the resultantoutput of the amplifier 21 by reason of the combined primary signalsource or pick-off 16 and the second signal source provided by thetapped leads 34, 35, amplifier 32 and the integrating network 41 fordifferent disturbing moments,

The transient curves in Fig. 3 illustrate the operation of the system orautomatic pilot whena disturbing moment of persistent character issuddenly applied to the system. The abscissa scale of this iiguirerepresentsr a time :axis and the ordinate scale indicates the input tothe amplifier 21 in volts. A constant signal K is shown illustrativelyin this figure as the signal required at the input to amplifier 21 tomaintain surface 10 in the position in which it is illustrated in Fig. lto balance a given persistent disturbing moment. Heretofore, signal Kwas provided necessarily by only thev output of pick-off 16. Inaccordance with theA described form of the present invention, thissignal is now substantially replaced by the output of network 41 shownby curve I which builds -up approximately to the value of signal K astime elapses.-

Curve I shows the change in the signal output of pick-off 16 withpassage ofy time. This signal, as shown by curve J, approaches a valueof zero because it is not'necessary for the craft to fly at the inclinedattitude or position necessary to continuously produce the relativelyconstant control signal or voltage K from only pick-olf 16. Curve K isthe summation of curves I and I. From Fig. 3, it will be understood thatwith disturbing moments that exist for only a fractional part of asecond, the integrating means or network 41 blocks the output signalfrom amplifier 32 so that the same does not effectively influence theinput to amplifier 21. The amplifier 32 and integrating network 41 forma means responsive to the signal of the displacement measuring means orpick-off 16 that provides a second signal in accordance with the effecton the object orcraft of only persistent disturbing moments. The signalsof pick-off 16 and network 41 are combined in a combining means in themanner shown in Fig. 3 to provide la substantially constant controlsignal as indicated at K for the automatic pilot or servo system thatcorrects for the effect of persistent disturbing moments. Y

In operation of the system with a persistent disturbing moment, theinitial control signal comes frompick-off 16.

,This moves elevator to the position shown in Fig. l

by which a counter-balancing moment for the disturbing moment isprovided. At such time, the crafthas moved about its pitch axis awayfrom its reference to an extent necessary to provide the required outputfrom the displaced parts of pick-off 16. As the signal from pick-off 16is continuous, with passage of time a signal builds up across resistor46 -as noted in curve I that is also effective to hold elevator 10 inthe position shown in Fig. l. The magnitude of signal I depends on thecharacteristics of amplifier 32. The rate at which the signal I buildsup depends on the characteristics of the network 41. As this signalbuilds up and approaches the total voltage K necessary to position theelevator,less signal is required from pick-off 16 and the craft movesabout its pitch axis in a `direction to return the craft to itsreference position.

Upon discontinuance of the persistent disturbing moment by retrimrningthe craft by adjusting either the trim tab or trim potentiometer 38,resistor 46 functions to discharge the condensers 44, 45 and reduce theintegration voltage to zero. Condensers 44, 45 charge with an outputfrom amplifier 32. When the amplifier output is reduced to a null point,any `charges in the condensers is dissipated' by resistor 46 which is inseries therewith and the output of the network 41 falls to zero rapidly.

In the form of the invention shown in Fig. 4A, the automatic pilotincludes displacement repeatback means in the form of a selsyn deviceindicated at 47. The rotor of device 47 is shown as positioned by shaft13 driven from motor 12. The stator of device 47 is shown in series inthe input circuit to amplifier 21 by means of leads 48, 50. With thesurface 10 away from a null orcentral position, the device 47 provides asignal whose magnitude depends on the magnitude of the displacement. Theoutput ofdevice 47 is fed to amplifier 21 in opposition -to thecontrolling input signal thereto from pick-off 16 and network 41.Displacement repeatback from the control surface to the system yinput isused to improve the systems fidelity. Fidelity is used in the sense ofinsuring that the system output, which in this case is position of thecontrol surface, is proportional to the error signal and that thedisplacement faithfully follows the input control signal with negligibletime lag relative to the craft response. Most control systems employthis form of repeatback since it produces better responsecharacteristics than many other types.

With a persistent disturbing moment ,requiring a sustained deliection ofthe control surface, the device 47 provides a continuous output. Thissystem consequently requires an initial displacement error signal frompick-off 16 that is greater than that required in the system of Fig. 4.However, the systemfunctions in the same manner as that heretoforedescribed, the corrective signal from resistor 46 oppositing the signalof the repeatback device 47 replacing the signal from pick-off 16.

` In the modified form of the invention shown in Fig. 5, thedisplacement error signal in the system due to a persistent disturbingmoment is entirely eliminated. As shown, the output of pick-ofi 16 isfed to a differential synchro device 52, in this instance, the rotor ofwhichA is positioned by motor 31 in the manner heretofore described in`connection with Fig. 4 so that the synchro device 52 provides a nulloutput across two legs of the differential synchro stator when thecrafts attitude about its pitch axis corresponds with the referenceposition desired of the same las defined by the gyro vertical 17. Inthis form of the invention, ythe signal of synchro device 52 is fed tothe amplifier 21 by way of a second synchro device 53.

In the present instance, the differential synchro device 52 is used tosynchronize the system to establish the desired reference attitude priorto engaging the control to monitor the aircrafts attitude. During thisinitial synchronizing phase switches 58 and 60 are located in the dottedline positions in Fig. 5. In this position, a reference signal outputfrom the differential synchro device 52 is nulled out by motor 31turning the synchro rotor through the control action of amplifier 32.The signal from synchro device 52 is fed to motor 31 by way of lead 61,switch 60, lead 61', amplifier 32 and switch 58. To insure there be nooutput signal from the second synchro device 53 at the time of engagingthe automatic pilot, switch 58 is thrown to the solid line position. Inthis position the second motor 54 turns the rotor of synchro device 53until the output voltage is zero. Sequence operation of switch 58 iscontrolled by the action of a conventional time delay control circuitwhich functions during the system warm-up period in a manner that allowssufiicient time for both reference signals to be nulled out before theautomatic pilot is engaged. When the automatic pilot is engaged, switch58 is retained in the solid line position shown in Fig. 5, and switch 60which has been in the solid line position to this point moves to thedotted line position. In this position the reference signal from synchrodevice 52, the null value indicating the craft reference attitude, isused to move the rotor of synchro device 53 to furnish a signal toamplifier 21 to produce motion of the control surface in the propersense to maintain the reference attitude. In this form of the invention,the integrating means responsive to the displacement measuring means, orpick-off 16, includes amplifier 32 and'motor 54. Amplifier 32 isconnected to the input to the servo amplifier 21 by switch 58, lead 61and motor 54. Motor 54 provides an element that with synchro device 53is equivalent to the integrating network shown in Fig. 1. Device 53driven by the motor 54 provides the second signal input to the servoamplifier as indicated at I in Fig. 3. The signal from synchro device 53due to the operation of the motor 54 provides a measure of persistentdisturbing moments only. The displacenient error signal from pick-olf 16is replaced in this arrangement by a signal from device 53 whose rotordriven by motor 54 is positioned by a signal fed thereto by way of lead55, switch 58, switch 60, amplifier 32, switch S8 and lead 61". It isunderstood from good control practice that the action of motor 54 duringthis operation is sufficiently slow in order not to affect the basicstabilizing effects of the system to short period transient disturbingmoments. The effective integral control action follows the same basiccontrol principles used to explain the systems shown in Figures 4 and4A.

A further control function can be employed in this control, which inthis case is provided from an altitude controller device 63. It will beunderstood that airspeed, Mach number, and similar controller functionscan also be used in this system. In this example, controller 63 is abarometric pressure sensitive device that generates electrical signalsproportional to departures from an established reference altitude. Thisdevice is similar to the pressure sensitive device shown and describedin U. S.

Letters Patent 2,446,546, issued August l0, 1948, to A. W. Meston. Thesensitive element or bellows 64 of the pressure operated device 63 isconnected to the spring suspended armature 65 of signal pick-off 66 bymeans of the electrically operated clutch 67. When the clutch is notengaged the moving end of the bellows rides free and the armature 65 iscentralized by the spring suspension producing zero signal output. Theelectromagnetic damping element 68 prevents undue movement of thearmatue 65 due to vibration of the whole unit which would produceunwarranted control action. Functional engagement of altitude controldevice 63 is accomplished by engaging clutch 67 thereby producing apositive connection of the signal pick-off armature 65 to the moving endof the bellows 64. Thus, changes in altitude which are registered bycorresponding changes in pressure result'in a direct movement of thepick-off armature generating an electrical output signal for controlpurposes. This signal is transmitted to the subsequent controllerelements over leads 69 and 70.

The altimeter device 63 thusly provides an altitude responsive meanswhich operating through the supplementary control system acts to movethe crafts control surface 10 in the proper sense to restore the craftto the altitude established at the time of engagement. The signalintroduced into the system by lead 70 is in the form of conventionalcontrol action which acts to change the pitch attitude of the craft tocorrect for short period altitude errors. The altitude control signalintroduced into amplifier 71 by lead 69 controls motor 62. Motor 62acting through the mechanical differential unit 59 turns the rotor ofsignal synchro device 53 creating an output control signal that is apart of the invention. It will be understood that the time required toeffect an altitude control correction through amplifier 71, motor 62 andr,

differential 59 will be in the order of 100 times longer than thecorresponding correction that would appear as a result of theintroduction of the signal into the system by lead 70. This form of theinvention would normally be used where direct control action, as in thisexample the introduction of the altitude error signal into servoamplifier 21 by lead 70, would not maintain the altitude within certaindesired limits. This system operates in the same manner described inconnection with the system shown in Figs. 4 and 4A where a sustainedaltitude error will create an additional control signal of a sustainednature that will slowly eliminate the sustained reference error. Theoutput of synchro device 53 may be fed to amplifier 21 by way of acircuit including a rate network 56 and a mixer 57. Mixer 57, as shown,also receives the output of pick-off 66 by way of lead 70 affordingconventional short-period altitude control for the craft.

Fig. 6 shows a further modification of the invention in an automaticpilot for aircraft provided with a trim tab surface 73 as well as themain control surface 10 for Controlling the craft about its pitch axis.This arrangesistent disturbing moments about the same axis.

ment also includes a displacement repeatback means in the form ofsynchro device 74 mechanically coupled through gearing 77 to shaft 1,3and to the control surface 10 similar to the device 47 shown in Fig. 4A.In this modified system, a portion of the output of generator 22 isemployed to energize the eld circuit of a motor 75 connected to trim tab73 by way of shaft 76, reduction gearing 79, drum 78 and cables 80. Thecharacteristics of motors 12 and 7S and associated gearing are such thatmotor 12 is primarily effective to move the main surface 10 to correctthe craft when displaced from its reference position about its pitchaxis due Ito transient disturbing moments and motor 75 is effective tomove the trim tab 73 to correct the craft for per- By use of the trimtab 73 in this arrangement, a sustained control effort from motor 12resulting from a persistent disturbing moment is balanced out. In thisform of the invention, the displacement measuring means is provided bypick-off 16. The selective means responsive to the signal of pick-off 16that provides a second signal in accordance with the effect on the craftof only persistent disturbing moments is formedof the signal synchro 81.The rotor of synchro 81 is positioned by the signal of pick-olf 16 bymotor 75 and shaft 76 which also determines the position of the trim tab73. The signal from synchro 81 provides the equivalent in this system tothe signal I indicated in Fig. 3. The signal is combined with the signalof the pick-off 16 to operate amplifier 21 yby way of the repeatbackcircuit in the same manner as shown in connection with the form of theinvention illustrated in Fig. 4A as will be described. As herein shown,the stator of device 81 is connected to the stator of synchro device 74by way of leads 82, 83 and 84. The signals from synchro device 81effectively change the normal or neutral position of the main controlsurface 10 to eliminate the need for a balancing signal input toamplifier 21 generated from pick-off 16. This balancing effect isproduced in the same manner that is employed in conventional synchrotransmitter and receiver units used in gun control systems. has shownthat corrective moments about the craft center of gravity are notproportional to deflection of the control surface. If the non-linearcharacteristics are known then the gear train used to drive synchrodevice 81 from trim tab motor 75 can be equipped with elliptical gearsor other suitable non-linear drives to introduce a balancing non-linearaction in the signal circuitry.

The operation of the automatic pilot system illustrated in Fig. 6 is asfollows. Assume that the craft is ying straight and level and is trimmedfor a particular craft loading in which the control surfaces are, forexample, streamlined. Assume also that this is the reference attitude ofthe craft as determined by the attitude reference provided `by gyro 17.Under this condition the gyro signal from pick-off 16 is zero and therepeatback signal from elevator repeatback synchro 74 is also zeroresulting in zero input voltage to control amplifier 21 and the elevatorservomotor 12. Now assume that the dis tribution of craft loadingchanges due, for example, to the release of bombs or consumption offuel, etc., which load change produces a sustained nose-down momentabout the craft, e. g., as illustrated at W in Fig. 1. Normal automaticpilot operation will deflect the elevator 10 through a y-up gyro signalto produce a balancing torque E. The aircraft now approaches level ight,i. e., the reference attitude, but will not reach this attitude sincethe elevator deflection must be maintained by a persistent torque on theelevator servo produced by a persistent gyro signal generated by thetendency of the craft to nose over due to the shift in craft loading. Asa results, a persistent voltage at the output of amplifier 21 isrequired to maintain the elevator deection. This persistent voltage isapplied to trim tab motor 75 which operates in an amount dependent uponthe length of time Experience persistent voltage is present andcorrespondingly deects trim tab 73 in a direction such as toaerodynamically unload the elevator surface and allow the persistentvoltage output from amplifier Z1 to go to zero. However, under theseconditions the gyro signal cannot go to zero because it must be presentto balance the feedback signal from repeatback synchro 77 on elevatorsurface 10. As a result the craft ilies along in an attitude, other thanthe reference attitude, as determined by the required signal from thegyro. In accordance with the present invention the aircraft may beallowed to return to its reference attitude. The trim ta'b surfacerepeatback signal from synchro 81 is used to shift the referenceposition of the main control surface by shifting the resultant voltagevector in the fixed field of elevator feedback synchro 74 therebyresulting in an effective shift of the neutral position of the elevator.This shaft will, of course, be in an amount proportional to thepersistent displacement thereof as measured by the trim tab deflection.In this manner the repeatback signal from elevator synchro 77 iseffectively reduced to zero and hence allows the persistent gyro signalto go to zero and in turn allows the craft attitude to return to itsoriginal reference attitude. At this point the craft is again flyingstraight and level in its reference attitude but has been automaticallyretrimmed for the new distribution of craft loading.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. An automatic pilot for aircraft having a main control surface formoving said craft about an axis thereof to correct for transient momentsthereof about said axis and a trim tab for moving said surface tocorrect the 'craft attitude for persistent moments about said craftaxis, a irst motor operatively connected to the main control surface, asecond motor operatively connected to the trim tab, rst means forproducing a signal in accordance with displacement of the craft from areference position about said axis and for supplying said signal to saidmotors, repeatback means for producing a signal in accordance with thedisplacement of the main control surface from a null position and forsupplying the same to said motors in a sense to oppose the signal ofsaid first signal means, third signal means for providing a signal inaccordance with the displacement of the trim tab from a null position,and means for modifying the signal of said repeatback means inaccordance with said third signal means.

2. In an automatic pilot for dirigible craft having a main controlsurface and a trim tab, motive means connected to said surface and trimtab, first signal means for operating said motive means in response todisplacement of the craft from a reference position, and second signalmeans for operating said motive means in accordance with thedisplacement of the trim tab from a null position and effective toreplace the signal of said iirst signal means upon occurrence of apersistent disturbing moment and thereby permit the craft to return toits reference position.

3. An object controlling servo system for controlling the position of anobject in accordance with both transient and persistent departuresthereof from a reference position so as to return said object to saidreference position comprising means for defining said referenceposition, control means positionable from a neutral positioncorresponding to said reference position and responsive to departures ofsaid object from said reference position for returning said object tosaid reference position after a departure therefrom, means carried bysaid control means and responsive only to a persistent departure of saidobject from said reference position for controlling the position of saidcontrol means whereby to controlthe position of said object inaccordance with said persistent departure, and means responsivev to theposition of said lastmentioned means for correspondingly shifting theneutral position of said control means in accordance with saidpersistent departure whereby to allow said object to return to saidreferenceposition. i

4. An automatic pilot for aircraft for controlling the attitude of saidaircraft in response to both transient and persistent departures thereoffrom a reference attitude comprising means for defining said referenceattitude, a first control surface positionable from a neutral positioncorresponding to said reference attitude and responsive to departures ofsaid craft from said reference attitude for returning said craft to saidreference attitude' after a departure therefrom, a second controlsurface car- 'ried by said rst control surface and responsive only topersistent departures in the attitude of said aircraft from thereference attitude for maintaining the position of said first controlsurface required to arrest said persistent departure, and meansresponsive to the position of said second control surface forcorrespondingly shifting the neutral position of said rst controlsurface in a direction and to an amount to eliminate said persistentdeparture and allow said craft to return to said reference attitude.

5. A servo system for controlling the position of an object inaccordance with both transient and persistent departures thereof from areference position so as to return said yobject to said referenceposition comprising means for defining a reference position and forsupplying a departure signal having both transient and persistentcharacteristics, first means positionable from a neutral positioncorresponding to said reference position and responsive to the transientcharacteristics of said departure signal for controlling the position ofsaid object whereby to` return it to said reference position after atransient departure therefrom, second means carried by said first meansand responsive only to the persistent characteristics of said departuresignal for controlling the position of said tirst means whereby tocontrol the position of said object in accordance with persistentdepartures of said object from said reference position, and meansresponsive to the position of said second means for shifting the neutralposition of said rst means in a direction and to an amount to eliminatesaid persistent characteristic of the departure signal from saidreference device whereby to allow said object to return to saidreference position.

6. Aircraft control apparatus comprising in combination, means fordefining a reference attitude for said aircraft including means forsupplying signals proportional to both transient and persistentdepartures in the attitude of said craft therefrom, a rst controlsurface positionable from a neutral position corresponding to saidreference attitude and responsive to said transient departure signalsfor controlling the attitude of said craft whereby to return the same tosaid reference attitude after a transient departure therefrom, a secondcontrol surface carried by said first control surface for controllingthe position of said rst control surface and responsive only to saidpersistent departure signal whereby to control the attitude of saidcraft in accordance with said latter signal characteristic, and meansresponsive to the position of said second control surface for shiftingthe neutral position of said rst controlsurface in a direction and to anamount to eliminate said persistent departure signal from said referencedevice and thereby permit said craft to return to said referenceattitude.

l1 controlling the position of said first surface in response topersistent departures of said craft from said reference attitude,attitude reference means for providing a signal corresponding todepartures of said craft from said reference attitude of both transientand persistent natures, a first servomotor responsive to a transientcontrol signal from said reference means for operating said firstsurface in accordance therewith, a second servornotor responsive to apersistent control signal from said reference means for operating saidsecond surface in accordance therewith, and means responsive to theposition of said second surface relative to said rst surface forshifting the neutral position of said first surface in a direction andto an amount to reduce the persistent control signal from said attitudereference means to zero whereby to allow said craft to return to saidreference attitude.

8. An automatic pilot for aircraft having a first control surfacemovable from a neutral position corresponding to a reference craftattitude for controlling said craft attitude about an axis thereof inresponse to transient departures of said craft from said referenceattitude and a second movable control surface movable from a neutralposition relative to the position of said first surface for controllingthe position of said first surface in response to persistent departuresof said craft from said reference attitude, attitude reference means forproviding a signal corresponding to departures of said craft from saidreference attitude of both transient and persistent natures, a firstservomotor responsive to both transient and persistent control signalsfrom said reference means for operating said first surface in accordancetherewith, means responsive to the displacement of said first surfacefor providing a feedback signal in opposition to said conuol signalswhereby to position said first surface in accordance therewith, a secondservomotor connected to receive said attitude reference signals andresponsive only to the persistent nature thereof for operating saidsecond surface in accordance therewith, and means responsive to theposition of said second surface relative to said first surface formodifying said feedback signal in accordance with a persistentdisplacement of said second surface whereby to shift the neutralposition of said first surface to thereby reduce said persistent controlsignal to zero and allow said craft to return to said referenceattitude.

9. An automatic trim control system for aircraft having a main controlsurface for controlling the attitude of said aircraft and an auxiliarycontrol surface carried by said main surface for aerodynamicallycontrolling said main control surface comprising means for defining areference craft attitude, first means positionable from a neutralposition corresponding to said reference attitude and responsive to saidreference means for positioning said main control Vsurface in accordancewith both transient and persistent departures of said craft from saidreference attitude whereby to return said craft toward said referenceattitude, and second means responsive only to persistent departures ofsaid craft from said reference attitude for positioning said auxiliarycontrol surface in an amount to aerodynamically maintain said maincontrol surface at a position to counteract said persistent departure,and means responsive to the operation of said second means for shiftingthe neutral position of said main control surface with respect to saidreference attitude responsive means in a direction and to an amount toeliminate said persistent departure from said reference attitude wherebyto allow said aircraft to return to said reference attitude` ReferencesCited in the file of this patent UNITED STATES PATENTS 2,545,343Conviser Mar. 13. 1951 2,594,326 MacCallum Apr. 29. 1952 2,723,089Schuck et al. Nov. 8, 1955 2,733,879 Noxon Feb. 7, 1956

